Currently, thin wires (bonding wires) having a wire diameter of 20 to 50 μm or so are generally used as bonding wires which bond an electrode on a semiconductor device and an external terminal together. A thermal compression bonding technique with the aid of ultrasound is generally used for bonding a bonding wire, and, a general-purpose bonding device and a capillary jig which allows a bonding wire to pass through the interior thereof for connection are used. The leading end of a wire is heated and melted by arc heating, a ball is formed by surface tension, and then the ball portion is compressingly bonded on the electrode of a semiconductor device heated within a range from 150 to 300° C., and the bonding wire is directly bonded to an external lead by ultrasonic compression bonding.
Recently, technologies related to the structure, material and connection of the semiconductor mounting technologies are rapidly diversified, and for example, in a mounting structure technology, in addition to currently-used QFP (Quad Flat Packaging) using a lead frame, new configurations, such as BGA (Ball Grid Array) using a substrate, a polyimide tape or the like and CSP (Chip Scale Packaging) are practically used, and a bonding wire which has improved loop characteristic, bonding characteristic, mass productivity, usability and the like is demanded. According to such bonding wire connecting technologies, in addition to ball/wedge bonding which is a currently major technique, for wedge/wedge bonding which is suitable for a fine pitch, a bonding wire is directly bonded at two portions, so that improvement of the bonding characteristic of a thin wire is demanded.
Materials which are to be bonded to a bonding wire are also diversified, and for materials of a wiring and an electrode on a silicon substrate, in addition to an Al alloy which is conventionally used, Cu which is more appropriate for fine pattern wiring is practically used. Moreover, an Ag plating, a Pd plating or the like is formed on a lead frame, and a Cu wiring is formed on a resin substrate or a tape, and a film of a noble metal element like gold or a combination of noble metal elements is often formed thereon. Correspondingly to such various kinds of bonding targets, it is demanded to improve the bonding characteristic of a bonding wire and the bonded portion reliability.
As the material of a bonding wire, 4N-group gold having a high purity (purity >99.99 mass %) is used. However, because gold is expensive, a bonding wire of another kind of metal that the material cost is inexpensive is demanded.
In order to meet the demand of a wire bonding technology, it is important to form a ball having a good sphericity when forming the ball, and to achieve a sufficient bonding strength at a bonded portion of the ball portion and an electrode. Moreover, to cope with temperature reduction of a bonding temperature and thinning of a bonding wire, and the like, it is necessary to achieve good bonding strength and tensile strength at a portion where a bonding wire is subjected to wedge bonding to a wiring on a circuit wiring substrate.
In a resin encapsulation step in which a thermal curing epoxy resin having a high viscosity is filled at a fast speed, there is a problem that a bonding wire is deformed and becomes contact to an adjoining wire, and to cope with the advancement of fine pitching, wire elongation, and wire thinning, it is demanded to prevent any deformation of a wire at the time of resin encapsulation as much as possible. Because of the improvement of the strength of a wire, such deformation can be controlled on some level, but practical usage cannot be realized if problems such that loop control becomes difficult and the strength at the time of bonding is reduced are not solved.
As the wire characteristic which satisfies such demand, it is demanded to acquire a comprehensive characteristic such that loop control in a bonding step is easy, the bonding characteristic to an electrode and a lead is improved, and any excessive wire deformation is suppressed in a resin encapsulation step after bonding.
To make the material cost inexpensive, to achieve a superior electrical conductivity, and to improve ball bonding and wedge bonding characteristics, a bonding wire made of copper (hereinafter, copper bonding wire) is developed, and this is disclosed in Unexamined Japanese Patent Application No. S61-99645. According to a copper bonding wire, however, the bonding strength is reduced due to oxidization of the wire surface, and the wire surface is likely to be corroded at the time of resin encapsulation. Those are the reasons that practical usage of a copper bonding wire is not accelerated.
According to copper bonding wires, in order to suppress any oxidization when melting a wire leading end to form a ball, bonding is performed while a nitrogen gas or a hydrogen-containing nitrogen gas is sprayed to the wire leading end. Currently, a nitrogen gas containing 5% of hydrogen is generally used as an atmosphere gas in forming the ball of a copper bonding wire. Unexamined Japanese Patent Application No. S63-244660 discloses a technique of connecting a copper wire to a copper or copper-alloy lead frame under the 5% H2+N2 atmosphere. Moreover, “Copper Ball Bonding for Fine Pitch, High I/O Devices”: P. Devlin, Lee Levine, 38th International Symposium on Microelectronics (2005), P. 320 to 324 discloses that because 5% H2+N2 gas can suppress any oxidization of a ball surface, such a gas is more desirable than N2 gas.
As a technology of suppressing oxidization of the surface of a copper bonding wire, Unexamined Japanese Patent Application No. S62-97360 discloses a bonding wire that copper is covered with a noble metal or a corrosion-resistant metal, such as gold, silver, platinum, palladium, nickel, cobalt, chrome, titanium, and the like. Moreover, from the standpoint of the ball formability and suppression of deterioration of a plating solution, Unexamined Japanese Patent Application No. 2004-006740 discloses a bonding wire structured as to have a core member mainly composed of copper, a dissimilar metal layer formed on the core member and made of a metal other than copper, and a coating layer formed on the dissimilar metal layer and made of an oxidization-resistant metal having a higher melting point than copper.
In practical usage of copper bonding wires, there are problems such that electrical resistance increases at a bonded portion of a copper bonding wire and an electrode under actual usage environment like high temperature and high humidity, and the bonding strength is reduced so that the long-term reliability is reduced. Such failures often occur when a bonded portion to an Al electrode which is widely used for normal semiconductor devices is encapsulated by a resin. It is expected that a corrosive reaction at a bonded portion of Cu and Al, and generation of a void are the factors of such failures. In the conventional usage environment of an IC, the problem of the bonding reliability of a copper bonding wire is hardly expected, but such a problem should be in consideration for a recent power IC, and a severe environment like an in-vehicle IC. Moreover, according to copper bonding wires, in comparison with conventional gold bonding wires, a wire surface is likely to be oxidized, the failure of the shape of a ball bonded portion and the reduction of the bonding strength are likely to occur.
As means for suppressing any surface oxidization of a copper bonding wire, it is possible to coat the wire surface with a noble metal or an oxidization-resistant metal. In consideration of the need of highly densification, miniaturization and thinning of the semiconductor mounting technology, the inventors of the present invention checked this technology, and confirmed that lots of practical problems have not been solved if a copper bonding wire (hereinafter, multilayer copper wire) having a multilayer structure that the surface of the bonding wire is coated with a metal other than copper is used with a conventional wire bonding technique.
For the conventional copper bonding wire having a monolayer structure (hereinafter, monolayer copper wire), a technology of forming a ball while spraying a ball formation gas to suppress any oxidization of copper, and connecting the ball portion on an electrode. As the ball formation gas, a gas mainly composed of nitrogen is generally used, and nowadays a nitrogen gas containing 5% hydrogen is most popularly used as a standard reference gas. When this bonding technique is applied to a multilayer copper wire, defects relating to the ball bonding characteristic occur. This results in reduction of the usage characteristic of a semiconductor in comparison with a case where a monolayer copper wire or a gold bonding wire which is currently popular is used.
There are practical problems that failures of the shape of a ball bonded portion and reduction of the bonding strength are likely to occur when a ball is formed at the leading end of a multilayer copper wire. As specific defective cases, formation of a flat ball having a poor sphericity, occurrence of misalignment that a ball is so formed as to be inclined relative to a bonding wire, remaining of a wire inside a ball which is not melted, and generation of an air bubble (blow hole) become problems in some cases. When such an abnormal ball is bonded on an electrode, off-centering deformation that a ball is formed but misaligned from the center of a wire, elliptical deformation, petal-like deformation and the like as shape-related failures that the sphericity becomes poor occur. This results in a reason that causes protrusion of a bonded portion from an electrode surface, reduction of the bonding strength, chip damage, and any failures on manufacturing management. Such initial bonding failures may cause reduction of the long-term reliability.
Not only to overcome such problems relating to the bonding characteristic of a ball, but also to practically use a multilayer copper wire which has a little actual usage achievement, the superiority of a performance which can replace a conventional copper bonding wire is needed. For example, it is demanded to improve the bonding strength in wedge bonding, to improve the bonding yield, or to reduce surface oxidization resulting in improvement of the shelf life of a copper bonding wire, more than the case of a monolayer copper wire.
From now, in order to accelerate the practical use of copper bonding wire, it is necessary to sufficiently cope with a thick wire having a diameter of greater than or equal to 50 μm which is not often used for the case of a gold wire for the power IC application, and to cope with a thin wire having a diameter of less than or equal to 20 μm which utilizes the high conductivity of copper, and regarding the characteristics, it is necessary to cope with more difficult demands, such as improvement of the bonding characteristic of a thick wire, small ball bonding with a fine pitch, low temperature bonding, and reverse bonding of a stacked chip connection.
Accordingly, it is an object of the present invention to overcome the problems of the conventional technologies relating to the practical use of the foregoing copper bonding wires, and to provide a bonding structure of a bonding wire which improves the formability and bonding characteristic of a ball portion, the bonding strength in wedge connection, and has a superior industrial productivity, and, a manufacturing method for forming the same.